KR100690508B1 - Austenitic stainless steel article having a passivated surface layer - Google Patents

Abstract

Austenitic stainless steel products are disclosed, preferably in the form of tubes. The product has a passivated surface layer, which in the case of the tube is on the inner surface of the tube. The passivated surface layer has Fe ₂ O ₃ and an oxide component having Cr ₂ O ₃ , Fe with zero valence and a metal component with Cr having zero valence. The ratio of the weight percentage of the oxide component to the weight percentage of the metal component exceeds 8: 1.

Description

Austenitic stainless steel article having a passivated surface layer}

1A and 1B are graphs showing surface components as a function of passivation time;

2 is a graph showing the ratio of metal to iron as a function of passivation time;

 3 is a graph showing the change in the ratio of Cr 2 O 3: Cr and Fe 2 O 3: Fe as a function of passivation time;

 4A is a graph showing iron binding energy scans showing relative iron oxide and free iron levels;

4B is a graph showing iron binding energy after 1 minute passivation showing a decrease in oxide and an increase in free iron;

5A is a graph showing chromium binding energy scan of passivating material showing relative oxides relative to free chromium;

5B is a graph showing the chromium binding energy scan of a material after 60 minutes passivation showing a decrease in free chromium;

FIG. 6 is a graph showing the combined energy scan of 60 minutes passivated material with significant residual free iron;

FIG. 7 is a graph showing depth profile using Auger electron spectroscopy of electro-polished and passivated surfaces; FIG. And

8 is a graph showing the depth profile of three different colored electro-polished surfaces showing color change as a function of chromium content.

The present invention relates to austenitic stainless steel products, in particular in the form of tubes, having a passivated surface layer.

In the manufacture of tubes made of austenitic stainless steel, in particular austenitic stainless steel, passivation in order to prevent the surface from oxidizing during the use of the tube, which would otherwise react with the environment encountered during use. Prevention is preferred. In particular, in the case of austenitic stainless steel pipes, in particular in the case of AISI type 316 stainless steel pipes, as used in the pharmaceutical industry, the inner surface is changed to an oxide form product which appears red during use. This phenomenon is generally referred to as "rouging". Such products can cause contamination of materials passing through the tubes during use in various industrial applications.

 It is therefore a primary object of the present invention to provide an austenitic stainless steel product, in particular a tube, having a passivated surface layer that does not lose upon exposure to an oxidative environment during use.

According to the invention, a stainless steel product, which may be in the form of a tube, has a passivated surface layer of Cr2O3 and Fe2O3, each of which is composed of Cr with zero valence and Fe with zero valence, respectively. The ratio of the oxide component to the metal component is at least 8/1.

Preferably, the stainless steel is austenitic stainless steel.

Preferably, the stainless steel is AISI type 316 austenitic stainless steel.

Preferably, the outer surface of the passivated surface layer will have a total Cr to Fe ratio of at least 1/1.

The passivated surface layer will have an overall ratio of Cr to Fe of at least 1.5 / 1 at the depth of the maximum oxygen concentration. The passivated surface layer is preferably composed of an electro-polish surface but may also be produced by mechanically polished surfaces such as swirl or belt polishing.

The reference to "total Cr to Fe ratio" includes Fe and Cr which appear in the oxide component.

The term "electro-polished" means a metallic polished surface produced by a combination of electrical action with an acid solution, one of which is phosphoric acid and the other usually sulfuric acid.

All components are expressed in weight percent unless otherwise indicated.

According to the invention, preferably the passivated surface layer is a mechanically polished surface obtained by electropolishing, electropolishing with oxidizing agents or treated with oxidizing agents. Therefore, the passivation process for obtaining a passivated surface layer according to the present invention is preferably carried out by exposing the surface to oxidants after it is otherwise polished, such as by electro-polish or otherwise by polishing. Is achieved. In this action, the surface is increased in chromium to iron ratio; Removal of surface roughness; Increase in oxygen penetration depth; Removal of contaminants such as occluded iron or removal of strain modified martensite; Removal of inclusions, in particular magnesium sulfide; And may be specifically adapted by removal of visible manufacturing defects.

During the passivation process, in part, the process occurring in the air within hours after the stainless steel surface is polished or otherwise polished, such as by electropolishing, chromium combines with oxygen to form an impermeable chromium oxide barrier. This prevents any further reaction under the passivation or barrier film. As the chromium content increases, the film is conditioned to be a better barrier. During the electropolishing period, iron and other elements on the surface are selectively removed in order to increase the chromium content on the surface. As a result, after electro-polishing, the chromium to iron ratio increases markedly on the passivated surface layer. As can be seen in FIG. 7, the average oxygen penetration depth is a measure of the depth of the passivated layer. In general, the deeper the oxygen penetration, the thicker the passivation layer and the more corrosion resistance the material has. However, this is only true if the oxide components are Cr₂O₃ and Fe₂O₃, that is, if the metal components Cr and Fe have both valences of zero and Cr₂O₃ and Fe₂O₃ combined so that the ratio of Cr₂O₃ to Fe₂O₃ is relatively high. Yes. This can be achieved by exposing the polished surface to an oxidizing agent such as nitric acid or citric acid for a period of time that is properly determined to fully react with Cr2O3 and Fe2O3. The composition change can be regarded as a function of the passivation layer as can be seen in FIG.

Passivation has a significant effect on the chromium to iron ratio in mechanically polished type 316L stainless steel pipe. Portions of the same tube were exposed to hot nitric acid for several passivation times and the passivation layer was analyzed using X-ray photoelectron spectroscopy (XPS). Changes in surface chemistry, particularly in terms of basic iron in the passivation layer, are very significant. There were significant differences in the Cr: Fe ratio and the ratio of elemental chromium to chromium oxide. Other elements exhibiting unusual aspects were silicon and molybdenum. More iron and chromium are present in the passivation layer in the mechanically polished tube than in the electropolished tube, suggesting that the surface of the mechanically polished tube may be more easily corroded.

Type 316L stainless steel is the material of choice for the high purity water (HP) and water for injection (WFI) systems in the pharmaceutical industry. For these systems two final conditions are used: electropolished and mechanically ground. The tube is usually required to specify ASTM A 270, which, in its current form, requires mechanically grinding regardless of the smoothness of the existing surface. Mechanical polishing takes one of two forms: swirl polishing or longitudinal belt polishing. Swirl polishing uses a rotary flapper wheel that moves up and down along the length of the tube to remove only a thin surface layer of material and create a "smeared surface". Longitudinal belt polishing uses an abrasive belt that moves along the length of the tube while the tube is rotating and uses an air bladder to pressurize the belt to remove surface material. This technique is a pioneer in electropolishing to remove significant amounts of material, 0.0006-0.0008 inches, and also to lower Ra levels (<8 micro inches or 0.2 micro inches). Both methods eliminate the common depth passivation layer produced in the manufacture of the stainless steel plates from which the tubes are made.

   Occasionally, discoloration of mechanically polished surfaces occurs, especially in hot and damp weather. This is seen in both types of mechanically polished tubes. This surface discoloration will appear from bright yellow to bright red. It can be easily removed by soaking in hot nitric acid and then washing with water. Once the tube has been acid treated, it does not discolor again as long as the treatment takes place at elevated temperature for a sufficiently long time.

Research has begun to determine what changes occur on the surfaces of mechanically polished tubes at various nitric acid passivation times. The concentration of the acid was specified at a specific temperature of MIL STD QQ-P-35 and ASTM A967-nitric acid 3, namely 120-140 ° F. (50-60 ° C.). These concentrations and temperatures provide the best results according to standard salt spray tests. In this study, the time at that temperature was varied and the surfaces were analyzed using Ray Photoelectron Spectroscopy (XPS). The results of the passivation study are as follows.

Reagent grade nitric acid was diluted to 20 volume percent (v / o) with deionized water and then heated to a predetermined 136 ° F. (58 ° C.). Five samples of mechanically polished tubes were immersed in this solution for 1, 5, 15, 30 and 60 minutes respectively. One sample was analyzed under “as polished” conditions. After washing and drying, the treated mechanically polished samples were each evaluated using XPS. There were no visible differences between the six samples. All had the same surface gloss.

X-ray photoelectron spectroscopy is one of the newer analytical tools available and is known as Electro Spectroscopy for Chemical Analysis or ESCA. During XPS, samples are irradiated with emitted photoelectrons analyzed for monoenergy soft X-rays and energy reactions. For this experiment a monochromatic A1 Kα X-ray was used at 1486.7 electron volts. These X-rays interact with atoms on the surface and emit photoelectrons. These optoelectronics occur within approximately 30-50 kHz of the surface and as a result have a kinetic energy expressed as follows:

                              KE = hv-BE-Φs

here:

KE is the kinetic energy;

hv is the energy of photons;                     

BE is the binding energy of the atomic orbital where electrons are generated; And

Φs is the spectrometer work function.

Each element and compound has its own set of binding energies. Therefore, XPS can be used to distinguish elemental concentrations of the surface being analyzed and to determine the binding energy of the surface type. This is a very useful function because it is possible to check the compositional change of the passivation layer as a function of passivation time.

Following each surface scan, the surface was subjected to collisions ("sputtering") with ionized argon to remove about 25 kV of material (or about 8 atomic depths), and the new surface was analyzed again. This continued until the maximum oxygen penetration depth was reached or until there were no more composition changes.

For each sample of constant depth, survey scans were made within an energy range of 1200-0 eV to determine the elemental composition. Then, for each element of interest, a narrow band of about 20 eV around the center peak was analyzed in high energy decomposition mode to determine the binding energy of the surface type. Peak transitions in XPS are considered a measure of covalent valence, and more ionic compounds, such as compounds between metals, do not transition or shift significantly from peak values of pure elements. The binding energy obtained for each element is compared with the theoretical standard value based on the standard or chemical bonds in the existing literature. The presence of overlapping, multiple bond energy, can make the distinction difficult. Handbook of Photoelectron Spectroscopy (JF Moulder et al., Physical Electronics, Inc., Eden Prarie, Minnesota, 1995) and Auger and Practical Surface Analysis by Auger and Data from X-Ray Photoelectron Spectroscopy, D. Briggs et al., J. Wiley & Sons, Chichester, England, 1983) was used to assign binding energy to the compounds.

The XPS system used for analysis is the Physical Electronics Model 5700. The binding energy values were corrected with carbon from atmospheric exposure, set to international standard, 284.7 eV. Quantitative figures for the data are obtained by using the sensitivity elements disclosed in the above-mentioned D. Briggs publications, which are based on the results calculated for pure elements. The analyzed information should at best be treated as semi-quntitative and should be used most appropriately only for comparison.

Since all samples were taken from the same tube and within 1 inch (25 mm) of each other, only one of the samples was analyzed as received after isopropanol cleaning to remove contamination from the handling. Each surface treated with acid was analyzed by XPS. In addition, the received samples and the 30 and 60 minute passivated samples were sputtered to determine the elemental composition and oxidation state as a function of depth.

Table 1 summarizes the surface chemistry observed for Type 316L stainless steel samples after several hours in hot nitric acid. The data show the atomic percentage composition of elements with atomic number 3 or more within 40 ms (12 atoms) of the surface. 1a and 1b graphically depict only atomic surface concentrations of metals as a function of passivation time.
Table 1: Surface composition of elements as a function of nitric acid passivation time Passivation time (minutes) C N O Na Mg Al Si P S Ca Cr Fe Ni Mo 0 41.8 2.4 39.9 0.4 - - 1.8 - 0.3 - 2.6 9.4 0.5 0.2 One 24.3 2.3 47.3 0.1 0.2 0.4 1.8 0.7 0.4 0.1 10.6 9.7 1.4 0.7 5 24.1 2.3 48.6 0.4 - 0.1 0.8 0.7 0.2 0.2 11.8 8.6 1.5 0.7 15 23.3 2.6 47.7 0.2 0.2 0.4 0.8 0.7 0.3 0.2 12.2 9.1 1.7 0.7 30 25.1 1.6 51.4 - - 0.3 0.9 0.8 - 0.1 13.0 5.7 0.7 0.3 60 28.8 1.8 49.9 - - - 1.1 0.4 - - 10.5 6.7 0.5 0.3

The data show that chromium and oxygen concentrations reached their maximum after 30 minutes of passivation and iron had the lowest. When the data is compared with the metal to iron ratio as shown in Table 2 and FIG. 2, the maximum Cr / Fe ratio occurs 30 minutes after passivation. For some unexplained reason, passivation at 15 and 60 minutes both shows a reduction in Cr / Fe ratio. Both Ni / Fe and Mo / Fe ratios reached their maximum at 15 minutes and began to decrease after 30 minutes of passivation.
Table 2: Ratio of important elements to iron as a function of nitric acid passivation time Passivation time (minutes) Si / Fe Cr / Fe Ni / Fe Mo / Fe 0 0.191 0.277 0.055 0.024 One 0.188 1.088 0.141 0.068 5 0.089 1.367 0.176 0.075 15 0.90 1.351 0.186 0.078 30 0.165 2.299 0.131 0.050 60 0.166 1.578 0.073 0.039

The 0, 30, and 60 minute passivated samples were sputtered with ionized argon and the elemental composition was determined as a function of depth. The data is summarized in Table 3 for the sample accepted. Table 4 is a 30 minute passivated sample and Table 5 is a 60 minute passivated sample.

Table 3: Composition as Depth Function for Polished Samples

Depth C N O Si S Ar Cr Fe Ni M 0 41.8 2.4 39.9 1.8 0.3 - 2.6 9.4 0.5 0.2 25 4.1 - 32.7 0.6 0.2 1.2 13.4 42.2 4.6 0.7 50 4.2 - 14.9 0.8 - 2.1 11.0 59.2 6.5 1.2 100 3.8 - 9.4 0.9 - 2.5 13.4 63.5 4.8 1.6 200 2.1 - 7.2 0.6 - 2.5 15.0 65.7 4.8 1.9 400 1.5 - 5.4 0.9 - 2.5 15.8 66.1 5.7 2.0 800 1.6 - 3.8 0.5 - 2.4 16.1 68.4 4.9 2.2 1600 1.4 - 3.1 0.3 - 2.5 16.6 68.8 4.8 2.2

Table 4: Composition as depth function of type 316L passivated with nitric acid for 30 minutes

Depth C N O Si Ar Ca Cr Fe Ni Mo 0 2.51 1.6 51.4 0.9 - 0.1 13.0 5.7 0.7 0.3 25 3.7 0.2 48.1 0.7 1.4 0.1 21.5 20.4 3.0 0.5 50 3.1 0.3 43.3 0.7 1.7 0.1 20.6 26.1 3.2 0.6 100 2.3 - 39.3 0.2 2.0 0.1 20.9 31.4 2.9 0.8 200 1.9 - 34.4 0.3 2.2 0.1 20.7 36.4 3.1 0.9 400 2.1 - 28.8 - 2.3 - 19.3 42.8 3.3 1.2 800 1.8 - 21.2 - 2.3 0.1 18.4 50.7 4.1 1.5 1600 1.9 - 11.6 - 2.4 0.1 17.4 59.9 4.8 1.9

The examination of specific binding energy peaks for each element indicates that both oxides and metals are present, ie, metals with zero valence. In the case of iron, oxides and elemental iron are present in significant amounts. This is especially the case for elemental iron at passivation times of less than 30 minutes. Table 6 and Figure 3 show the ratio of iron and chromium to their respective oxides.

This data indicates that iron oxide abruptly decreases after 1 minute and continues to fall until chromium oxide is reached to reach a saturation point, which is somewhere between 15 and 30 minutes. After 30 minutes, both ratios increase, although the increase rate is greater for chromium oxide than iron oxide. This indicates that the surface is more passivated by long exposure to hot nitric acid.

Table 5: Composition as depth function for type 316L passivated with nitric acid for 60 minutes

Depth C N O Si P Ar Ca Cr Fe Ni Mo 0 28.8 1.8 49.9 1.1 0.4 - - 10.5 6.7 0.5 0.3 25 8.6 0.3 49.8 0.9 0.3 1.2 0.1 17.2 19.6 1.7 0.4 50 5.6 0.4 47.8 0.8 0.1 1.5 0.2 17.1 24.0 2.1 0.5 100 4.0 0.3 45.2 0.5 0.1 1.7 - 18.1 27.7 2.1 0.5 200 4.0 0.3 45.2 0.5 0.1 1.7 - 18.1 27.7 2.1 0.6 400 3.0 - 29.1 0.1 - 1.7 - 17.2 35.8 2.3 0.8 800 2.0 - 33.7 - - 1.8 0.1 17.3 41.3 2.7 1.1 1600 2.1 - 24.3 - - 1.8 - 17.7 49.2 3.4 1.4 3200 1.8 - 12.8 - - 2.1 - 17.1 60.0 4.3 1.9

Table 6: Ratio of iron and chromium oxide to metal during various passivation times

0 min 1 minute 5 minutes 15 minutes 30 minutes 60 mins Fe ₂ O ₃ / Fe ⁰ 1.0: 1 0.5: 1 0.4: 1 0.3: 1 1.5: 1 3.8: 1 Cr ₂ O ₃ / Cr ⁰ 3.2: 1 4.5: 1 4.5: 1 4.5: 1 8.5: 1 13.0: 1

The passivation treatment of mechanically polished 316L type stainless steel appears to be necessary to improve its corrosion resistance. Mechanical polishing destroys the passivation layer formed during the manufacture of the strips and tubes. The passivation layer is very thin and is on the order of 50-400 mm 3 or an atomic thickness of 12-150. Although swirl polishing does not remove the appropriate amount of metal, the passivation layer is destroyed as evidenced by surface oxidation. When these oxidized surfaces are immersed in hot nitric acid, the color disappears, indicating removal of iron oxides. Thus, passivation following polishing is a necessary task.

The most dramatic change in surface chemistry occurs after only one minute in hot nitric acid, during which time the Cr / Fe ratio of the surface changes from 0.26: 1 to 1.1: 1. This ratio may vary depending on the type of analytical instrument used. Auger electron spectroscopy (AES) tends to show lower values than XPS. Many of these changes appear to dissolve surface iron oxides as shown in FIGS. 4A and 4B. Carefully examining the binding energy curves for iron and chromium, the metallic chromium (valence is zero) gradually decreases as passivation time increases, as shown in FIGS. 5A and 5B. Chromium oxide indicates an increase. However, as shown in FIG. 6, even after 60 minutes of passivation, metallic iron remains an important kind. As a comparative example, the electropolished material shows little metallic iron, which means that it will have better corrosion resistance.

The mechanism for passivation appears to be involved in the gradual oxidation of chromium as the first step. Once free chromium is consumed, iron begins to form its oxide. Atmosphere-forming iron oxides that prevail in the material as received are rapidly dissolved in hot nitric acid and metallic iron remains the predominant species for up to 30 minutes, where the amount of oxides eventually exceeds the amount of metallic iron. True passivation does not appear to occur until all of the metallic elements are converted to oxides. For mechanically polished materials it will be over 60 minutes to passivate in hot nitric acid.

The following is the conclusion from this experimental work.

1. Mechanically polished surface chemistry of type 316L results in quite dramatic changes during passivation. Iron is reduced, as is silicon, nickel, and molybdenum. Oxygen and chromium both increase. The Cr / Fe ratio increases with passivation time.

2. The passivation mechanism may be controlled by the oxidation of metallic chromium to trivalent oxide. Only when chromium is satisfied does iron begin to form trivalent oxides that can be detected.

3. Even after 60 minutes passivation in hot nitric acid, a clear metallic iron peak is still present, indicating that more passivation may occur.

Except in very limited fields such as the pharmaceutical and semiconductor industries, electropolishing has not been recognized as a means of providing an improved finish. Electropolishing, together with a high gloss surface comparable to chromium plating, is recognized as a means of producing surfaces that are free from accidental iron contamination, extremely smooth, and free from surface defects. It is also recognized that the electropolished surface has improved corrosion resistance compared to the machine polished surface.

With the advent of specialized analytical instruments, it was possible to accurately measure what was happening on the surface. Auger Electron Spectroscopy (AES) was the first of these technologies and was first introduced only 30 years ago. Some time later, "sputtering" with ionized argon was developed, which allowed AES to measure the composition as a function of distance from the surface. 7 shows a typical AES depth profile of an electropolished surface. The main problem with AES is that only elements are reported, and their molecular form is not reported.

Another very useful analytical technique developed at about the same time is Energy Dispersive Spectroscopy (EDS). This is also an elemental analysis method and can be used with scanning electron microscopes such as microprobes to identify the composition of small particles such as inclusions in steel.

Newer technology, also known as X-ray photoelectron analysis or XPS, chemical analysis electron spectroscopy (ESCA) uses X-ray instead of electrons. This method has the advantage of identifying molecular types. There are some differences between the values reported for XPS, AES and EDS. The reason for this is not fully understood, but is entirely due to the difference in the depth of analysis, the size of the spot, and the type of spectra generated. A comparison of these three analytical techniques is shown in Table 7.

Table 7: Comparison of Analytical Technologies

Technology Auger Electron Spectroscopy (AES) X-ray photoelectron spectroscopy (XPS) Energy Dispersive Spectroscopy (EDS) Probe beam Electronic X ray  Electronic Detection beam Auger electronics Optoelectronic X ray Element range 3-92 2-92 5-92 Detection depth 30Å 30Å 1 μm Detection limit 1X10 ^-3 1X10 ^-4 1X10 ^-5 accuracy 30% 30% 10% Identification of the organization? no part no Identify chemical state? part Yes no

XPS can be used with sputtering to identify the chemical state of an element and to obtain a depth profile, thus allowing evaluation of surface treatments that improve corrosion resistance. For this reason, XPS was used as the main evaluation tool. The main means of comparison was the Cr / Fe ratio. Other ratios of interest included the ratio of oxides Cr ₂ O ₃ / Cr ⁰ : Fe ₂ O ₃ / Fe ⁰ . The latter ratio makes it possible to follow the relative oxidation rate for different metals, so it is probably optimal for describing passivation techniques.

Other experimental work was performed to test electropolishing and passivation as a means of improving corrosion resistance. Moreover, the effect of orbital welding on the surface has been considered as an improved property.

As explained and shown above, mechanically polished surfaces have a very low Cr / Fe ratio. This is indicated by the data shown in Table 8. As also described and shown above, "air passivation" does not improve the Cr / Fe ratio. Leaving the air passivated surface to use without passivation can accelerate "rouging" in applications of high purity water. Proper passivation will significantly improve the Cr / Fe ratio in all cases.

Table 8: Comparison of Cr / Fe ratio of tubes using various polishing techniques

Polishing method Cr / Fe ratio Electropolishing, no passivation 0.82 Longitudinal belts, no passivation 0.28 Rotary swirl, no passivation 0.33

Electropolishing is simply reversed electroplating. The process involves pumping a solution of concentrated sulfuric acid and phosphoric acid through the interior of the tube during direct current application. The metal is dissolved from the tube (anode) and the cathode will be plated if the chemical nature of the solution has not been equilibrated to dissolve the metal so fast that the metals are plated. Since oxygen is released at the surface of the tube, the resulting passivation layer has a high Cr ₂ O ₃ / Fe ₂ O ₃ ratio. The result is a very smooth surface with high gloss. A complete description of such a process is disclosed in "Manufacturing Electropolished Stainless Steel Pipes" (J. C. Bergberg, TPJ-Tubes, and Pipe Journal, September / October 1998).

Normally, the surface finish is measured with a frofilometer and is normally expressed as Ra or average roughness. However, roughness alone is not sufficient to explain the true nature of the surface. Using a scanning electron microscope with a profilometer is the best surface analysis.

Typically, electropolished tubes with surface roughness of up to 10 microinches (0.25 micrometers) or up to 15 microinches (0.38 micrometers) will be used industrially. Depending on how the surfaces were prepared prior to electropolishing, there are differences in how to achieve these two finishes. In general, as described above, two methods of mechanical polishing are used to prepare the surface. For the smoothest surface, the inner tube surface is polished using a longitudinal belt. This removes most of the metal from the ID surface below the depth of defects caused in manufacturing. When electropolished, surface finishes of 2 to 5 microinches (0.05 to 0.12 micrometers) are not uncommon. Another method of mechanical polishing utilizes a rotating flapper wheel that produces a swirl finish. When electropolished, a surface finish in the range of 8 to 13 microinches (0.20 to 0.33 micrometers) is achieved. Swirl polishing results in a "smeared" surface with little removal of metal, so that scarring of the surface is rarely removed. Bright surfaces that are worked in a high cold state and are hydrogen annealed will result in the same surface finish. In both cases the Cr / Fe ratio will be about the same.

As indicated by the experimental work described above, passivation has the effect of introducing oxygen into the surface layer and dissolving other elements, leaving chromium and iron as two major surface metals. Both carbon and oxygen are present at high concentrations. Some carbon and oxygen are from occluded carbon dioxide. Carbon is higher on machine polished surfaces than on electropolished surfaces.

The investigations thus far included 20% nitric acid at 50 ° C and 25 ° C, and hydrofluoric acid of 1% 20% nitric acid at 50 ° C and 25 ° C. Passivation time varied with solution and temperature. Two other passivation treatments were planned for later studies: 10% citric acid + 5% EDTA and 5% orthophosphoric acid.

The effect of color tinted electropolishing surfaces on the composition of the passivation layer has been studied. Gold staining appears to oxidize all Cr ⁰ and Fe ⁰ to trivalent oxides and shows an extremely high Cr / Fe ratio. When the color changes to blue, iron begins to form Fe ₃ O ₄ , which is also expressed as Fe ₂ O ₃ FeO, and the chromium content drops as shown in the depth profile of FIG. 9.

Recent passivation studies include swirl polished surfaces, swirl polished + electropolished surfaces, and longitudinal belt + electropolished surfaces. The largest Cr / Fe ratio, 4.04, was achieved by swirl polishing after only 20 minutes in hot nitric acid, and the Cr / Fe ratio was reduced to 3.15 after 30 minutes. Table 9 compares the various passivation treatments for different starting materials.

Table 9: Effect of various passivation treatments on Cr / Fe ratio

process No passivation 20% HNO ₃ at 50 ℃ 20% HNO ₃ at 25 ℃ 20% HNO ₃ + 1% HF at 50 ° C 20% HNO ₃ + 1% HF at 25 ℃ 0 min 20 minutes 30 minutes 10 minutes 30 minutes 5 minutes 10 minutes 20 minutes 5 minutes 10 minutes 30 minutes Swirl (only) 0.33 4.04 3.15 2.95 2.09 2.59 1.52 1.15 2.58 2.45 1.58 Swirl + EP 1.33 2.23 1.94 2.34 2.04 1.82 1.52 2.07 2.13 2.08 2.20 Belt + EP 0.82 1.69 1.91 2.05 2.01 2.16 1.77 2.15 1.75 2,24 2.20

In each case where the Cr / Fe ratio decreases as the passivation time is extended, the amount of chromium metal to chromium oxide and free iron to iron oxide increases. This means that the surface layer is melting and the substrate is regaining proper Cr / Fe balance. This is logical with the use of hydrofluoric acid, because it is a hydrofluoric acid that readily attacks chromium.

Circumferential welding of swirl polished 316L stainless steel tubes was analyzed using XPS. In this study, weld beads, slag deposits on weld beads, and black oxides in the heat affected zone were analyzed. Data is shown in Table 10.                     

Table 10: Composition of Circumferential Welding Components

element Cr Fe Ni Mo Mn Si Al Ca Cr / Fe Welding beads 4.3 37.8 0.8 0.3 4.6 0.5 - - 0.11 Slag point 1.9 5.3 - - 2.0 4.0 1.4 12.9 0.36 HAZ oxide 16.2 3.0 - 0.1 21.5 2.7 - 0.3 5.40 Base metal 14.9 23.6 - 0.1 3.0 0.8 - 0.4 0.63

The results indicate that non-passivated welds have a very low Cr / Fe ratio. Ideally, the Cr / Fe ratio should be 1.0 or higher to have reasonably good corrosion resistance. There was no depth profile using XPS at these sites, but based on EDS analysis, the content of chromium increased with depth. Chromium was highly variable depending on the sample, perhaps depending on whether the electron probe analyzed delta ferrite or austenite. The results are consistent with other EDS analytical work, where the weld surface always shows more manganese and less chromium.

Similarly, slag patches are consistent with other findings. Slag is seen as an accumulation of incomplete gas ranges or inclusions in the steel that allows oxidation of the weld pool. In this case, the slag point appeared to come from the content in the steel and the steel was reduced to calcium and aluminum.

The black oxide sites across the heat affected zone have the highest chromium levels and the lowest iron among the sites analyzed. Compared with actual corrosion failure cases in the field, the black oxide appears to remain intact and acts as a crevis former with crevice corrosion occurring below the black oxide. This is because a lot of chromium makes these black oxides corrosion resistant, so galvanic corrosion allows the surface to adhere underneath the oxide.

Some important observations have shown differences between mechanical polishing surfaces, electropolishing surfaces, passivated surfaces, and circumferential weld surfaces. These are as follows.

1. The machine polished surface has all the elements appearing in the alloy and in roughly the same proportions.

2. Electropolished and passivated surfaces show no molybdenum and almost no nickel. Although silicon is variable and its valence morphology may change in the case of electropolished surfaces, only two elements that are really important are chromium and iron.

3. Electropolished surfaces tend to have a greater depth of oxygen penetration than passivated surfaces.

4. Passivated surfaces in a reasonable time appear to have a higher surface Cr / Fe ratio, but not the depth of oxygen penetration.

5. The Cr ₂ O ₃ / Cr ^o ratio appears to control the passivation process.

6.Fe ₂ O ₃ / Fe ^o The ratio can have a greater impact on passivation than the Cr / Fe ratio, and therefore on corrosion resistance. Fe ⁰ The lower is the more stable the passivation layer.

7. Circumferential welding has very low chromium and iron-rich surfaces. Manganese also rises.

8. The black oxide over the area affected by the heat of the circumferential weld is very high in chromium, low in iron, and generally associated with crevice corrosion in the field.

9. Slag deposits that appear occasionally on the circumferential weld surface appear to be low melt refractory components generated from oxides in steel or weld pools.

These observations mean that the passivation layer can actually be a crystal in nature. The closest crystal form is chromite spinel, which has a general composition of (Fe, Mg) O. (Cr, Fe) ₂ O ₃ . These crystals have oxygen atoms disposed on the face-centered cubic lattice (Dana et al., Textbooks in mineralogy, John Willy & Sun, New York, 1951), and thus fit the crystal lattice of austenitic stainless steels. In addition, because of the composition of the crystal, the lack of certain elements in the surface layer of the passivated and electropolished material is accounted for, and provides a reason why the passivation process takes time in the oxidizing solution to form crystals. Iron-rich surfaces will not form adequate crystals and will therefore lack chemical stability.

Since the composition of the circumferential welding is low in chromium, the surface crystal resulting therefrom will be hematite (Fe ₂ O ₃ ) or magnetite (Fe ₃ O ₄ ), all of which have no corrosion resistance. Therefore, the surface must first be passivated with acid to dissolve excess iron, and then allow chromium to become the dominant element.

The black oxide over the heat affected area has a general composition of chromite, FeCr ₂ O ₄ or FeOCr ₂ O ₃ . The composition can vary considerably, but in all cases there is a lot of chromium. This gives good corrosion resistance to the crystals in the oxidizing medium, which is probably much better than the metals it encompasses. This leads to conditions for galvanic corrosion (crevis corrosion) and explains the type of corrosion observed in systems that resulted in poor gas ranges during welding. The only correction is chemical dissolution of the oxide, which typically consists of nitric acid plus hydrofluoric acid, which must passivate the entire system. However, this treatment can destroy the electropolished surface.

The following has been concluded and established from these additional experimental work.

1. The inside of stainless steel pipe can be adjusted to increase the service life. The two most common systems are electropolishing and acid passivation. In each case, the Cr / Fe ratio needs to reach or exceed 1.0 to achieve the best corrosion resistance.

2. The amount of free iron in the passivation layer is important for the stability of the layer. If the free iron exceeds iron oxide, the film will become unstable, which can lead to breakage during use.

3. The passivation will reach an optimal Cr / Fe ratio in a relatively short time, after which it seems to reverse itself.

4. Some properties of the passivation layer suggest that it may be crystalline and takes on the properties of chromite spinel.

5. The circumferential weld surface contains a lot of iron and manganese, but very little chromium, suggesting that the weld surface is poor in corrosion resistance.

6. Black oxide, which can cover the weld zone affected by heat, is very high in chromium and low in iron. This suggests that the oxide is achromate and has very good corrosion resistance.

7. The slag point that sometimes appears on the weld surface is an accumulation content from steel. Under conditions of poor gas range this slag point can be the oxidation of silicon, iron and chromium inside the molten weld pool.

Other embodiments of the invention will be apparent to those skilled in the art from the description and practice of the invention disclosed herein. It is intended that the specification and examples of the invention be considered only by way of example, with the true scope and spirit of the invention being defined by the following claims.

The stainless steel product according to the present invention is excellent in corrosion resistance and can be used for surface treatment of pipes.                     

Claims (20)

An austenitic stainless steel product having a passivated surface layer, the passivated surface layer comprising Cr ₂ O ₃ , an oxide component having Fe ₂ O ₃ , Fe having a valence of zero, and a valence. 10. An austenitic stainless steel product, comprising: a metal component having Cr having a zero zero valence, wherein the ratio of the weight percent of the oxide component to the weight percent of the metallic component is greater than 8: 1. delete The method of claim 1, The austenitic stainless steel is austenitic stainless steel product, characterized in that the AISI type 316. The method of claim 1, The exposed surface of the passivated surface layer has an overall Cr weight percentage: Fe weight percentage ratio of at least 1: 1, wherein the austenitic stainless steel product. The method of claim 1, And said passivated surface layer has an overall Cr weight percent: Fe weight percent ratio of at least 1.5: 1 at a depth of maximum oxygen concentration. A stainless steel tube having an inner surface layer passivated on an electro-polished inner surface of an austenitic stainless steel tube article, the passivated surface layer having Fe ₂ O ₃ and Cr ₂ O ₃ . A metal component having an oxide component, Fe having zero valence, and Cr having zero valence, wherein the ratio of the weight percent of the oxide component to the weight percent of the metallic component is greater than 8: 1. Austenitic stainless steel tube, characterized in that. delete The method of claim 6, The austenitic stainless steel is austenitic stainless steel tube, characterized in that the AISI type 316L. An austenitic stainless steel product having a passivated surface layer on a mechanically polished surface of an austenitic stainless steel product, the passivated surface layer comprising an oxide component having Fe ₂ O ₃ and Cr ₂ O ₃ ; A metal component having Fe having a valence of zero and Cr having a valence of zero, wherein the ratio of the weight percent of the oxide component to the weight percent of the metallic component is greater than 8: 1. Austenitic stainless steel. delete The method of claim 9, The austenitic stainless steel is austenitic stainless steel product, characterized in that the AISI type 316. The method of claim 9, The exposed surface of the passivated surface layer has an overall Cr weight percentage: Fe weight percentage ratio of at least 1: 1, wherein the austenitic stainless steel product. The method of claim 9, And said passivated surface layer has an overall Cr weight percent: Fe weight percent ratio of at least 1.5: 1 at a depth of maximum oxygen concentration. The method of claim 6, And the exposed surface of the passivated surface layer has an overall Cr weight percent: Fe weight percent ratio of at least 1: 1. The method of claim 6, Said passivated surface layer having an overall Cr weight percent: Fe weight percent ratio of at least 1.5: 1 at a depth of maximum oxygen concentration. An austenitic stainless steel tube having an inner surface layer passivated on a mechanically polished inner surface of an austenitic stainless steel tube article, the passivated surface layer having Fe ₂ O ₃ and Cr ₂ O ₃ . A metal component having an oxide component, Fe having a valence of zero, and Cr having a valence of zero, wherein the ratio of the weight percent of the oxide component to the weight percent of the metallic component is greater than 8: 1. Austenitic stainless steel tube. delete The method of claim 16, Austenitic stainless steel tube, characterized in that the stainless steel is AISI type 316. The method of claim 16, The exposed surface of the passivated surface layer has an overall Cr weight percent: Fe weight percent ratio, wherein the austenitic stainless steel tube is at least 1: 1. The method of claim 16, Said passivated surface layer having an overall Cr weight percent: Fe weight percent ratio of at least 1.5: 1 at a depth of maximum oxygen concentration.

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